Article: Ecology of a Grazing Ecosystem: The Serengeti
Authors: McNaughton, S.J.
Journal: Ecological Monographs, 55(3) 1985, pp. 259-294
In previous posts, we have analyzed research dealing with spatial heterogeneity on the landscape. Sometimes spatial heterogeneity is a desirable management objective, and can be achieved through management, such as rotating fire annually through paddocks or through grazing planning.
But spatial heterogeneity is also a natural result of a landscape that is edaphically, climatologically, and topographically variable. These natural differences occur both through time and space. In many instances, human managers have failed to grasp this most fundamental of natural principles at a landscape scale.
S.J. McNaughton’s research shows that animal behavior in unmanaged grassland ecosystems is often a response to spatial and temporal heterogeneity. During their annul migration, wildebeest and other grazers are constantly reacting to uneven precipitation patterns across a climatologically stochastic landscape:
Large wildebeest and gazelle herds arrived on the Serengeti Plains within 3 days of the first significant rainfall and remained there as long as precipitation promoted grass growth. They kept the vegetation in a grazing lawn throughout the season so that leaf tissue was almost all that was available.
Natural cycles determine the movements of migratory mega-herds on a landscape scale:
…nomadic herds move at the beginning of the dry season to grasslands that have sustained low grazing intensities during the wet season. Large herds enter those mid-grasslands at their peak biomasses, graze them heavily, then move onward.
Those familiar with the Serengeti mega-herd migrations are aware that animals move from the high precipitation north and into the brittle southern plains during the rainy season. These mega-herds move in a clockwise migratory pattern from north to south and back north again over a 6 to 8 month period, giving birth to and rearing their young in the process. Video evidence shows animals are highly sensitive to precipitation patterns, and as note above, will follow rainfall in search of green swards.
These descriptions may give one the impression that precipitation patterns are merely uneven over large geographic distances as climate regimes change with latitude and topography. While this is true, it does not paint a complete picture of what is actually happening. Precipitation patterns are also uneven at relatively short distances, creating spatial and temporal heterogeneity at macro and micro scales; animals seem to have built-in behavioral mechanisms to deal with these random natural phenomenon. As McNaughton observed:
The lack of correlation between productivity patterns of three stands on the Serengeti Plains separated by distances of 4-10 km emphasizes the low predictability of productivity pulses. Productivity ranged up to a maximum of 40 g/m2/d in such pulses, so the food potentially available to ungulates can be substantial, but its occurrence in space and time is highly variable. The ability of grazers to track such productivity pulses was documented by a close relationship between gazelle density and primary production over the duration of such a pulse.
Population densities in time and space are determined by precipitation, and ultimately by primary productivity:
When primary productivity increased due to showers, the gazelle population increased; when green biomass decline between showers, the population decreased. Localized areas of higher primary productivity, therefore, act as grazing foci.
Holistic Management is based on premises gleaned from careful observation of nature. But we must not forget that our managed ecosystems are inherently artificial. Fencing, vaccinations, domestic livestock, submersible pumps, these are all technological innovations that are common place on most ranches. In our haste to privatize land and commodify its output, we may have inadvertently inhibited the ecological forces responsible for maintaining healthy grasslands. One unintended consequence is our inhibition of the natural migratory behavior of grazing ungulates. In North America, bison herds exhibited similar behaviors to the mega-herds of the Serengeti:
Historical accounts of the American bison suggest that they were fully as mobile as the Serengeti migratory fauna. Rapid movements, sometimes over large distances, are a characteristic behavioral pattern of undomesticated animals that evolved in grassland ecosystems.
Even at small scales, our inhibition of natural animal behavior may have consequences for the health of the land:
Deterioration of agricultural grazing lands may be less a consequence of stocking density than of the reduced responsiveness of a confined fauna to the temporal and spatial dynamics of grassland ecosystems.
While we may never fully restore these grassland ecosystems, we have available to us the tools we need to more accurately mimic the natural processes that maintain healthy grasslands within the socioeconomic context of modern ranching. Some guidelines can help us improve our planning based on grassland ecology. First, precipitation is critical to grassland productivity. Every drop of rain must be conserved through enhanced management. Animal impact can break soil crusts and the keyline plow can improve water infiltration for maximum conservation.
Regardless of soil condition, precipitation is inherently uneven across the landscape, especially in brittle environments. The larger the management unit is in size, the more likely one is to encounter uneven precipitation patterns from one year to the next. Grazing planning can be used to mimic the natural behavior of grazing animals, creating grazing foci in areas of high precipitation and permitting deferred recovery of temporally arid zones. This type of planning may also improve animal performance.
As our understanding of these processes evolves, it may become possible to build landscape level grazing corridors that more accurately mimic natural grassland migrations. Mega-herds with multiple owners may move through these corridors via negotiated arrangements with landowners as grass farming becomes a separate but complimentary profession to animal husbandry. Currently, this possibility is wishful thinking considering our current socio-economic and cultural context; however, with time, both economics and biology may trump culture, and people may begin to realize the benefits of restoring these natural cycles.